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Langmuir 1995,11, 551-554
Adsorption of (y-Aminopropyl)triethoxysilane on Silica from Aqueous Solution: A Microcalorimetric Study P. Trens, R. Denoyel," and J. Rouquerol Centre de Thermodynamique et de Microcalorimitrie du CNRS, 26, rue d u 141bme RIA, 13331 Marseille Cedex 03, France Received February 7, 1994. I n Final Form: September 7, 1994@ Adsorption of (y-aminopropy1)triethoxysilane(APS)on silica from aqueous solution was studied in situ by determining the differential enthalpies of displacement (microcalorimetry)and the adsorption isotherms (chromatographic type procedure). It is shown that, in the low concentration range (less than 0.2% in weight), the ethanol produced from the hydrolysis of the silane is not adsorbed whereas silanetriol (the other product of the hydrolysis) does not form polycondensates. The major role of the amino function in adsorption is demonstrated by the similar behavior of APS and of a much simpler aminated molecule, i.e. propylamine. "his similarity is seen from both the microcalorimetric results and the influence of pH on adsorption. Nevertheless, the partial irreversibility of APS adsorption (evidenced here by liquid flow microcalorimetry and not observed for propylamine) seems to show and measure the role of siloxanebonds in the adsorption of APS on silica.
Introduction The adsorption of organosilane molecules at the solid/ liquid interface has been the subject of numerous investigations in the past due to their widespread use in various FTIR,20p21,23 t e c h n o l o g i e ~ . ~For - ~ ~example, NMR,19,23,25 and calorimetry22have already been used to study the interaction of ( y-aminopropy1)triethoxysilane (AF'S) with silica in order to get information about both the conformation of the adsorbed molecule and the nature of the adsorbate/adsorbent bond. This is actually a complex system since this molecule is able to interact with the
* Author to whom correspondence should be addressed.
Abstract published in Advance A C S Abstracts, December 15, 1994. (1)Plueddemann, E.In Silane CouplingAgents;Plenum Press: New York and London, 1991;pp 221-248. (2)Iler, R. K. In The Chemistry ofSilica;Wiley and Sons: New York, 1979. (3)Johannson, 0.K.; Stark, F. O.;Vogel, G. E.; Fleischmann, R. M. J . Composite Mat. 1967,1, 278. (4)Kang, H. J.;Blum, F. D. J . Phys. Chem. 1991,95,9391. (5)Van der Voort, P.: D'Hamers, I. G.; Vansant, E. F. J . Chem. SOC. Faraday Trans. 1991,86(22),3751. (6)Severin, J. W.; Van der Wel, H.; Camps, I. G. J.; Baken, J. M. E.; Vankan, J. M. J . Surf. Interface Analysis 1992,19,133. (7)Kurth, D. G.; Bein, T. J . Phys. Chem. 1992,96,6707. (8)Tripp, C. P.;Hair, M. L. Langmuir 1992,8, 1961. (9)Azzopardi, M. J.; Arribart, H. J . Adhesion 1993,6 , 230. (10)Wang, M. J.;Wolff, S. Rubber Chem. Technol. 1992,65, 715. (11)Kallury, K. M. R.; Cheung, M.; Ghaemmaghami, V.; Krull, U. J.; Thompson, M. Colloids Surf. 1992,63,1. (12)Suzuki, Y.; Maekawa, Z.; Hamada, H.; Kibune, M.; Hojo, M.; Ikuta, N. J . Mat. Sci. 1992,27,6782. (13) Blum, F. D.; Meesiri, W.; Kang, H. J.;Gambogi, J. E. J.Adhesion Sci. Technol. 1991,5,479. (14)Strazielle, C.; De Mathieu, A.-F.; Daoust, D.; Devaux J . Polymer 1992. ----, 33. - - , 4174. (15)Grime, J. K.; Sexton, E. D. Anal. Chem. 1982,54,902. (16)Maciel, G. E.;Sindorf, D. W. J . A m . Chem. SOC.1980,102,7606. (17)Maciel, G. E.;Sindorf, D. W.; Bartuska, V. J . Chromatogr. 1981, 205,438. (18)Sindorf, D. W.; Maciel, G. E. J . Phys. Chem. 1982,86,5208. (19)Ishida, H.; Koenig, J. L. J . Colloids Interface Sci. 1978,64,555. Koenig, J. L. J . Colloids Interface Sci. 1978,64,565. (20)Ishida, H.; (21)Chiang, C.-H.; Ishida, H.; Koenig, J. L. J . Colloid Interface Sci. 1980,74,396. (22)Kelly, D. J.;Leyden, D. E. J . Colloid Interface Sci. 1991,147, 213. (23)Chiang, C.-H.; Liu, N.-I.; Koenig, J. L. J . Colloid Interface Sci. 1982,86,26. (24)Leyden, D. E.;Shreedhara Murthy, R. S.; Atwater, J. B.; Blitz, J. P. Anal. Chim. Acta 1987,200,459. (25)De Haan, J. W.; Van den Bogaert, H. M.; PonjeB, J. J.; Van de Ven, L. J. M. J . Collloid Interface Sci. 1986,110,5910. @
~~
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silica surface not only by its amino group but also by the silanol functions resulting from the hydrolysis. The infrared spectrum of adsorbed indeed shows that the amino function is affected by surface silanols, whereas NMR result^^^,^^ evidence the formation of siloxane bonds either between the silane molecule and the surface silanols, leading to bi- or tridendate structures, or between the adsorbed silane molecules themselves, by cross-linking. These results lead to a picture where the silane molecules are fmed to the surface by either one or two function^.^^*^^ Moreover, it was shown that the relative proportion of aminohilica and siloxane bonds depends on the thermal treatment of the the siloxane bonds being favored by an increase of the duration and/or temperature of the treatment. Nevertheless, most of the above studies were carried out on the adsorbed phase after removal of the solvent (under various drying conditions: with or without heating, under an inert atmosphere, or under vacuum) so that little is known about the initial interaction between the silane molecules and the silica surface. This is the gap we wish to fill in the present study, taking advantage of the fact that adsorption microcalorimetry, together with the determination of adsorption isotherms, lends itself to insitu measurements, whatever the amount of solvent present.26
Experimental Section Materials. We used two silane molecules: (y-aminopropy1)triethoxysilane and [3-(methacryloxy)propylltrimethoxysilane (MPS),provided by Aldrich. The adsorptionof propylamine(PA) (from Fluka)was also studied in order to evaluate the influence of the amino function of AF'S. Acids and bases (HC1 and KOH from Fluka) in the form of pretitrated solutions were used to modify the pH. Acetic acid and barium acetate (purissimagrade from Fluka) were used to prepare a pH 4.2 buffer medium for
MPS solutions. The silica is a porous sample (X-015 grade supplied by 1.B.F). Its nitrogen BET surface area is 25 m2 g-l and its mean pore radius 150 nm. Particle size ranges between 40 and 100 pm. Methods. Adsorption isotherms. Adsorption isotherms were determined by the conventional solution depletion method in test tubes. The equilibrium concentrations were detected by either refractometry (APS or PA) or UV spectrometry (MPS). However, in the case ofAPS,a specific analytical procedure was (26)Trens, P. Ph.D. Thesis, Marseille, France, 1994.
0743-7463/95/2411-0551$09.00/0 0 1995 American Chemical Society
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552 Langmuir, Vol. 11, No. 2, 1995
t
Peak area
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Curve 5
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,
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,
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.
000s
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Figure 1. Chromatographic peak area of APS solutions as a function of concentration: curve 1, calibration curve analyzed with a chromatographic column; curve 2, calibration curve analyzed without a chromatographic column; curve 3, calibration result for curve 2-curve 1; curve 4, supernatant curve analyzed with a chromatographic column; curve 5 , supernatant curve without a chromatographic column. For concentrations lower than 7 x molkg-’, curves 1 , 4 , and 5 are separated for the sake of clarity; they are in fact fully coincident.
Figure 2. Adsorption isotherms ofAPS, MPS, and PA on silica at 25 “C. The pH values at the plateau are 9.5, 4.2, and 10, respectively.
and 5 shows that alcohol alone is present in the supernatant) and also of the nonadsorption of the alcohol molecule (the overlapping of curves 1 and 5 shows that initial and final concentrations of alcohol are equal). Microcalorimetry. The enthalpies of displacement of the solvent by the solute (a term more appropriate and accurate, in needed. The APS molecule is indeed hydrolyzed in aqueous this case, than “enthalpies of adsorption” or of course “heats of solution following the reaction adsorption”, the use of which ought to be discouraged) were determined with a microcalorimeter as already described? an NH,(CH,),Si(OEt), 3H,O isothermal differential microcalorimeter, using heat-flow meters and immersed in a thermostated bath controlled within K. NH,(CH,),Si(OH), 3EtOH It can be used with two different procedures: either a batch mode, where a mother solution is added stepwise to a stirred As a consequence, an APS aqueous solution is a mixture of suspension of the adsorbent, or a liquid flow mode, where the silanetriol molecules and ethanol. Moreover, silanetriol moladsorbent is held in a small column through which is successively ecules are able t o condense in polysiloxanes of various s i ~ e . ~ z ~ J ~flushed the pure solvent, solutions of increasing concentration, Although it is generally admitted that 0.15% is a threshold and the solvent again (in order to study the reversibility of concentration under which polycondensates do not form,’ we adsorption). The two procedures are used here and provide found it safe to try to separate the contribution of each species complementary information. In most cases it was possible to (silanol, alcohol, condensates) to adsorption. For this purpose, determine pseudodifferential enthalpies of displacement which we developed a chromatographic procedure able to separate these are more sensitive (than integral enthalpies) to any change in solutes from the supernatent. The setup is composed of a HPLC the adsorbed phase during its formation.28 Dilution enthalpies pump (Gilson 3021, a Rheodyne valve with a 200pL loop, a column of stock solutions used for adsorption were also determined in containing silica (the same as that used for adsorption experiorder to correct the values measured during the adsorption ments), and a refractometer (Waters R403). experiments for the heats of dilution.28 The supernatent from a test tube (or a calibration solution) is sampled with a syringue and injected into the chromatographic Results and Discussion system through the loop ofthe Rheodyne valve. The mobile phase is a KOH water solution (pH 9.4). Under these conditions, As indicated in the Experimental Section, the ethanol silanetriol molecules are fully retained by the column: the surface molecules produced by the hydrolysis of the APS molecule of the peak observed on the refractometer recording is indeed do not adsorb on silica. Therefore, in the following text, equal to that obtained with an ethanol solution prepared at the “APS adsorption” will have to be understood as trisilanesame alcohol concentration as in the APS solution, assuming a triol adsorption. The adsorption isotherm ofAPS on silica complete hydrolysis. Moreover, when the APS concentration is is presented in Figure 2. A preliminary kinetic study has above 0.15% in weight, a second peak is observed (at longer shown that adsorption is over after 1h (no further change retention times) which could correspond to the presence of u p to 4 h). This is an adsorption isotherm of the higholigomers. This is a confirmation of the figure given by Plueddemann,’ but we have not made further experiments in affinity type (the slope at the origin is pratically infinite) that direction (i.e., determination of oligomer content) since the with a well-defined plateau at 2.3pmol m-2. For the sake second peak not only depends on the concentration but also on of comparison, we have plotted, in the same figure, the the solution aging. In most cases, solutions used here have a adsorption isotherms of MPS and propylamine (PA). The concentration lower than 0.15% in weight. Finally, the total MPS solutions were analyzed by UV spectrometry, which procedure is the following (Figure 1). is here specifically sensitive to the silanetriol molecule or (a) Calibrationsolutions are analyzed as a function of the its oligomers. It seems logical to extend the observations initial APS concentration with a column (curve 1) and without made in the case of APS and to assume that the ethanol (curve2). For a given concentration,the signal differencebetween formed by the hydrolysis of MPS is not adsorbed. We curves 2 and 1only corresponds to silanetriol molecules, and one can get a straight calibration line for silanetriol by plotting this observe that the adsorption of MPS is negligible as difference as a function of the concentration (curve 3). compared with that of APS; the kinetics of adsorption are (b) Supernatants (after adsorption has occurred in the test also very different: 4 h were needed to achieve the tubes) are analyzed in the same manner, with the silica column thermodynamic equilibrium in the case of MPS. This (curve 4) and without (curve 5 ) . Curves 4 and 5 are plotted as result may look quite strange in view of the results a function of the initial APS concentration in the test tubes before adsorption. The difference between curves 4 and 5 for a given (27) Davy, L.; Denoyel, R.; Rouquerol, J. J . Calorimbtrie et Analyse initial concentration allows one to calculate the final concentraThermique; Association FranGaise de Calorimetrie et Analyse tion in silanetriol by using calibration curve 3. It is worth noting Thermique: Marseille, 1990;Vols. XX-XXI (proceedingsof Calorimetry that on a large range of initial concentrations, curves 1 , 4 , and and Thermal Analysis, held at Clarmont-Ferrand,May 1990). 5 overlap: this is an indication of the total retention of the (28) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J . Colloid Interface silanetriol molecules in the column (the overlapping of curves 4 Sci. 1990, 136, 236.
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Langmuir, Vol. 11, No. 2, 1995 553
A Microcalorimetric Study of APS Adsorption
0
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Figure 3. Surface concentration of AF'S at the plateau (rmax) Figure 5. Integral enthalpies of displacement (Adpmofwater from silica by APS (two successive experiments on the same versus pH at 25 "C. 80
2
substrate).
,
I
of adsorption is similar to that observed for alkyl amine^.^^ The latter molecules adsorb by electrostatic interaction between the SiO- surface sites and the RNH3+form of the amino function. The surface concentration of these sites, resulting from the dissociation of surface silanols, increases with pH,l whereas the RNH3+ concentration decreases: the combination of these two opposite trends leads to a maximum (Figure 3 h 3 0 In fact, the surface charge of a silica gel in the range 3-7 is very small.2 We have verified this point on our own silica by potentiometric titration (surface charge density lower than 0.1 pmol m-2 a t pH 7). Now, when the pH decreases, it is surprising that APS does not adsorb by formation of a siloxane bond. It may be due to the formation of an intramolecular bond between the RNH3+ and the SiOH function1 which would prevent the molecule from reacting with the surface. Nevertheless, the fxation of the amino function on the silica is not the only explanation since APS can be adsorbed at least twice more than PA. As quoted in the Experimental Section, microcalorimetric experiments were carried out either in a batch mode or in a liquid flow mode. They both provide the same displacement enthalpies, but the liquid-flow mode also provides important information about reversibility. As evidenced by the two curves in Figure 5 which give the displacement enthalpies for two successive adsorption experiments (with intermediate desorption),the adsorption ofAPS is (ca. 40%)irreversible (at least on the time scale of our experiments, i.e., with a desorption lasting around 5 h). Under the same conditions, propylamine is reversibly adsorbed. This suggests that APS is not adsorbed only by the amino function. According to the literature,lJ9 another possibility of fixation of the APS molecule is the formation of a siloxane bond by condensation between silanetriols and surface silanols. The amount of irreversible adsorption could be a measure of the amount of silane molecules fixed by at least one siloxane bond. This result also shows the catalytic role of the amino function in the condensation reaction: its direct involvement in the adsorption process is necessary to get the condensation. (APSis not a t all adsorbed in pH ranges where the NH2 group is not adsorbed, whereas the adsorption of MPS is negligible.) This reaction, as mentioned by Iler,2would lead to a weak variation of enthalpy. This could explain the slight decrease of displacement enthalpies as the coverage increases in the case of APS (i.e., the experimental heat is divided by the adsorbed amount of molecules fixed by different kinds of interaction) whereas the enthalpies of displacement by PA are nearly constant (i.e., the experi-
o o
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04
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UX
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IS
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Surlace concentration I p o l in-2
Figure 4. Differential enthalpies of displacement (Adpih) of water from silica by PA and APS at 25 "C as a function of
surface concentration.
published in the literature, but it must be borne in mind that the solutions we use here are much more diluted and are free from polycondensates, the behavior of which could be very different. The PA adsorption isotherm also shows a lower plateau than that of APS (1pmol m-2). It must be emphasized that PA and APS adsorption isotherms were carried out a t pH 9 whereas MPS was adsorbed at pH 4.2. The height of the APS plateau versus pH is given in Figure 3. The amount adsorbed is negligible for pH below 6, as in the case of PA, and presents a maximum at about pH 9, in agreement with the results of Naviroj et al.,29who found such a maximum at pH 10.5 (however they obtained a very high surface concentration, 45 pmol m-2, which may correspond rather to the formation of a deposit during drying than to an adsorption). A comparison with MPS at high pH is not possible since this molecule is then insoluble. A more striking evidence of the similar behavior of APS and PA is provided by the displacement enthalpies, as shown in Figure 4: the curves are relatively close to each other specially in the low coverage range. In the case of APS,several experiments were carried out with different concentrations of the stock solution injected in the calorimetric cell containing the silica suspension: between 0.1% and 1%in weight, the enthalpies measured change less than 5%, suggesting a minor influence-if any-of the oligomers which could appear during the aging of the solutions. It is remarkable that the enthalpy of displacement obtained here a t low coverage (-40 k J mol-l) is very close to that calculated by Kelly and Leyden (-44 k J mol-l) by an indirect method (titrationby an acid22). This similarity between APS and PA suggests that the APS molecule is adsorbed by its amino function, at least at the beginning of adsorption. This could explain that the pH dependence (29) Naviroj, S.; Culler, R.; Koenig, J. L.; Ishida, H. J.Colloidlnterface Sci. 1984, 97,308.
(30) Somasundaran, P.; Ananthapadmanabhan, K. P. Solution Chemistry Surfactants; Mittal, K. L., Ed., Plenum: New York, 1979; VOl. 2, p 777.
Trens et al.
554 Langmuir, Vol. 11, No. 2, 1995 mental heat is divided by the adsorbed amount of molecules fxed by one kind of interaction). It is then interesting to consider three kinds of adsorbed APS molecules: (1)molecules adsorbed by the amino function, (2) molecules adsorbed by a siloxane bond with a surface silanol or with another irreversibly adsorbed molecule, and (3) molecules adsorbed by both the amino function and the siloxane bond, as drawn in ref 23, for example. It is difficult to say, from our experiments, whether each type of adsorbed molecule corresponds to a unique type of site on the surface or whether rearrangements occur as the equilibrium concentration increases. For example, a molecule first adsorbed in the type 1configuration could switch to type 2 at a higher equilibrium concentration.
Moreover, the proportion of each kind of adsorbed molecule could change with time, like in the case of dried samples, as has been shown after outgassing and heating the above systems: the observed increase ofthe proportion of siloxane bonds23is consistent with the fact that the siloxane bond formation, as followed by adsorption microcalorimetry, is less exothermal than the aminohilica bond formation (Le.,the replacement of a NHdSiOH bohd by a siloxane bond is endothermal).
Acknowledgment. The authors wish to thank Saint Gobain Recherche (Aubervilliers)for the financial support of this research and, especially, P. Chartier and E. Dallies for fruitful discussion. LA940131B
